1. Field of the Invention
The present invention relates to the field of display systems, and more particularly solid state light source based display systems having a native gamut that are to display images from image data having a target gamut.
2. Prior Art
FIG. 1a illustrates a typical spatially modulated projection system. Central to most spatially modulated color projection systems (see “Projection Displays,”, E. H. Stupp et al., John Wiley and Sons Ltd., 1999), such as micro mirror or liquid crystal cells on silicon (LCOS) projectors, is a light pipe, which includes a white light lamp 110 and a color wheel 120. The color wheel 120 usually contains three different types of filters for selectively passing spectrums of red, green and blue {R, G, B} primaries. More recently, the color wheel also provides a fourth clear filter which is used selectively to reproduce a predefined set of colors, especially for grays (see U.S. Pat. No. 6,910,777). This four color primary system resembles the CMYK color system of printers and leads to higher brightness and contrast specifications. This concept has been generalized to provide five or more primaries to enhance the reproduced color gamut (see U.S. Pat. Nos. 5,526,063 and 6,769,772).
However, in all these display systems, at any particular instance of time, only one of the color primaries; e.g. red, green or blue, can be turned on. Hence, the color properties of the display system are dictated completely by the chrominance and luminance properties of the color filters used in the color wheel 120 (see “Visible Laser and Laser Array Sources for Projection Displays”, Jansen et al., Proc. of SPIE Vol. 6135, 2006). For example, the color gamut of a display system cannot be changed to match a predefined standard gamut. As a result, the display device performance depends heavily on the quality of the color primaries filters used on the color wheel 120 and how close these primaries, in terms of their chrominance, to the predefined standards or target gamut color primaries. This can be a severe restriction especially since the number of standards are defined and redefined at a much faster rate today.
More recently, the projection industry, like any other display industry, has been driven to produce compact, low-power, high-longevity projectors without sacrificing the display quality. This saw the advent of projectors illuminated by solid state light (SSL) sources, such as light emitting diodes (LED) and laser diode (LD) (see U.S. Pat. Nos. 7,101,049 and 7,334,901, the disclosure of which is hereby incorporated by reference). SSL sources can provide bright and saturated colors with orders of magnitude higher longevity. FIG. 1b illustrates a spatially modulated projection system that uses SSL sources 140. Each of the color primaries in the light pipe of the projection system illustrated in FIG. 1b is generated by a SSL 140 comprised of a single or array of SSL devices of a specific color (see U.S. Pat. Nos. 7,101,049, 7,210,806 and 7,334,901 and “Visible Laser and Laser Array Sources for Projection Displays”, Jansen et al., Proc. of SPIE Vol. 6135, 2006). Furthermore, since multiple primaries are provided by multiple light sources, the light pipe does not need include any color wheel. The most important feature of the projection system architecture illustrated in FIG. 1b in the context of this invention is that, unlike projectors with a light bulb and color wheel, in projectors with multiple SSL such as the projection system architecture illustrated in FIG. 1b, more than one color primary can be turned on simultaneously as described in prior art (see U.S. Pat. No. 7,334,901). Furthermore, unlike the arc lamps used in most common projection systems such as that illustrated in FIG. 1a, which typically require several tens of seconds to be turned on, SSL can be switched on and off in much less than a microsecond.
In conventional spatially modulated projectors, such as illustrated in FIG. 1a, since the color properties of the display are completely dictated by the physical properties of the color filters, color properties can be manipulated only in a limited fashion. For example, matching the display system color gamut to predefined standards, such as NTSC 210 or HDTV 220 illustrated in FIG. 2 (which shows various color gamut plotted in (u′,v′) chromaticity color space), white point, or brightness are important performance parameters for any projection-based product in the display market. However, the color gamut generated by conventional spatially modulated projectors, such as illustrated in FIG. 1a, is defined by the chromaticity coordinates of its red, green and blue filters and cannot be changed without changing the color filters 120 or the light bulb 110. Typically the white point of conventional spatially modulated projectors, such as illustrated in FIG. 1a, can be changed only in a limited manner by biasing the displayed image pixels grayscale values of each of the color primaries at the cost of reduced contrast, dynamic range, wall-plug efficiency, and overall achieved brightness. Furthermore, often the controls to change these properties are not independent of each other, resulting in a very difficult calibration procedure which would suffer from convergence problems, especially if multiple properties, such as brightness and white point, are being optimized together.
In order to assure display system compliance with color property standards parameters (such as color gamut, brightness and white point), the display industry has to follow strict quality measures when manufacturing the light sources 110 and color filters 120 . For example, if the color gamut resulting from the color filters 120 does not cover the NTSC gamut 210, sophisticated gamut mapping methods need to be instrumented and still may not provide the required visual quality when a wider color gamut light sources such SSL sources 140 are used as illustrated in FIG. 2. As illustrated in FIG. 2, the color gamuts 250 and 260 provided by SSL sources 140 are typically much wider than typical commercial display systems color gamuts, such as NTSC 210 or HDTV 220. When the native gamut of the display system is much wider than the target gamut, such as when the SSL based projection display system architecture illustrated in FIG. 1b, is used targeting commercial display color gamut such as NTSC 210 and HDTV 220, special color mapping techniques are instrumented to limit the native gamut of the display system to the target gamut. Besides wasting the potentially higher luminous flux offered by the typical wider gamut 250 and 260 of the SSL sources, these techniques are often severely non-linear and often need a custom fit to each particular SSL device.
The hierarchical multicolor primaries multiplexing system of this invention make use of the capability of multiple color primaries on-cycles simultaneity offered by SSL-based projection display system (see U.S. Pat. No. 7,334,901) to remove all these rigidities. Using the hierarchical color primaries multiplexing system of this invention, a display system that confirms to standard target gamut, white point and brightness can be easily provided for a wide variety of SSL sources which may not strictly adhere to the predefined standards. Furthermore, the hierarchical color primaries multiplexing system of this invention can also be used to maximizing the display system capabilities, in particular brightness and wall-plug efficiency, while completely adhering to the required target gamut and white point specifications.
Central to many projection display systems is the spatial light modulator (SLM), such as the micro mirror and LCOS devices (see U.S. Pat. Nos. 5,535,047 and 4,596,992). In such projection systems, such as those illustrated in FIGS. 1a and 1b, that uses a reflective type SLM device, such a micro mirror or LCOS devices, the reflective state of each of the SLM device pixels can be set digitally based on the desired on/off state of each of the pixels forming the digital image. The projection image is formed by sequentially modulating each of the display system color primaries spatially using the SLM device with the image pixels grayscale data of each color primary. The pixel grayscale data for each color primary, which is typically expressed as multiple bit words, is typically converted into serial bit stream using pulse width modulation (PWM) technique. The PWM bits are used to set the SLM device pixel on/off state. Typically the digital image data associated with each color primary are modulated by the SLM device sequentially in a temporal multiplexing fashion. This temporal multiplexing of each of the primaries in conjunction with PWM technique is used to create spatial modulation 1-bit planes for each color primary which are loaded into the SLM device to set the on/off state of each of its pixel to express the different grayscale value of each image pixel color primary (see U.S. Pat. No. 5,280,277). However, when SSL sources are used, such as in the architecture illustrated in FIG. 1b, the color primaries generated by the SSL source would typically have different chromaticity characteristics than the color gamut required for most commercial display systems as illustrated in FIG. 2. As a result the conventional temporal multiplexing of the native color primaries (meaning the native color primaries generated by the SSL devices) of the display systems cannot be used. Furthermore, as explained earlier, the color gamut mapping schemes currently used in SSL-based projection systems are inflexible and inefficient. The objective of this invention is, therefore, to describe a hierarchical multicolor temporal multiplexing system that can be used in SSL-based projection systems to improve color quality and stability as well as the efficiency of the display system.